I’m sorry you’re facing this difficult task, Gagajo. 
It’s not something we’ve faced, as my poor sister-in-law dealt with my MIL’s house - we live nearly 600 miles away and my Dh was working abroad when MIL died.
My own mother is an inveterate clearer-outer so there won’t be much to go through there, although at 94 she’s still healthy and robust! She had got rid of everything of my dad’s by the day after his funeral, whereas MIL still had FIL’s gardening clothes hanging in the hall, nearly 25 years after he’d died. I know there will be no memories to be found in my mum’s house, although I believe there is one piece of jewellery she wants me to have, a brooch given to her own mother by her first fiancé who was killed in WW1. I don’t think I’ve ever set eyes on it, though.
We have been decluttering our own house as we will downsize at some point. I randomly hit upon a method of deciding whether to chuck or keep those more personal items. Would I ever use/look at/listen to etc that item again? Also, would any of our dc ever be interested? If it had no purpose, practical or aesthetic, then out it went.
It was easier doing it that way than simply asking myself do I want to keep that, because I’d probably keep it all if I could.